Method of making an injection molded propeller

ABSTRACT

A method of making an inexpensive injection molded propeller for mounting on a drive shaft. The drive shaft is attached to the hub of the propeller by a chemical and thermal shrink fit and by deformation due to the rotational forces of the propeller thereby producing a positive locking hub on the drive shaft. The propeller may be used, not only for remote piloted missiles and aircraft, but can also be used with different types of boat inboard and outboard drives, vehicles and equipment requiring a propeller having a positive locking system for securing the propeller or the like to the drive shaft.

BACKGROUND OF THE INVENTION

Heretofore, there have been different types and methods used forfabricating propellers. These methods include pattern machining usingnumerical control equipment and forging or a combination of thesetechniques. The methods requiring post machining for finishing andpolishing the propeller greatly add to fabrication expense. In order tokeep expenses at a minimum some economies are necessary. For example,the twist angle of the hub to the tip of the blade is not consistent incurrent propellers because of stress relief occuring during themachining process. This occurs primarily in metal propellers but alsooccurs to a more limited extent to composite wooden propellers. Inaddition, limitations of current machining requires non-optimaltransition from the hub of the propeller to the blade. This comprisessmooth air flow which reduces propeller efficiency. Propellers incurrent use are fabricated using compromises between what is requiredfrom an aerodynamic point of view and what can be fabricated from apractical cost point of view. In designing a producible propeller, bynecessity, fabrication costs as related to functional value is a primaryconsideration.

The use of injection molding techniques permits, for the first time,primary consideration to be given to the aerodynamic requirements of thedesign of the propeller with no requirement to conform to conventionalfabrication technique limitations. The injection molding manufacturingapproach with materials which have not previously been used forprepellers has produced a highly efficient propeller at a low cost. Acomparable two-bladed wooden propeller costs normally over $100.00. Afour-bladed metal propeller cost approximately $800.00. While the costof injection molding four-bladed propellers with shaft extension can bemanufactured at a cost below $50.00.

U.S. Pat. No 3,670,382 to Keehan, U.S. Pat. No. 3,423,700 to Hardy, U.S.Pat. No. 2,659,444 to Stanley, U.S. Pat. No. 3,470,604 to Zenick, U.S.Pat. No. 3,438,116 to Stengle, Jr., U.S. Pat. No. 2,679,913 to Scott,U.S. Pat. No. 2,058,618 to Patzig, U.S. Pat. No. 1,388,657 to McDonaldet al and U.S. Pat. No. 3,480,373 to Talbot all disclose different typesof blade design, propeller design and methods of securing articles ofmanufacture through the use of thermoplastic. None of the above patentsdisclose the unique features of the injection molded propeller andmethod of making this molded propeller for producing a positive lockpropeller mounted on a drive shaft.

SUMMARY OF THE INVENTION

The injection molded propeller provides a propeller wherein the driveshaft can be attached to the hub of the propeller both by a chemical andthermal shrink fit and by deformation due to the rotational forces ofthe propeller thereby providing a unique positive locking hub on thedrive shaft.

The injection molded propeller comes from a pre-polished mold therebyeliminating the need to polish the propeller. The consistency of the airfoil and propeller shape of the mold propeller eliminates therequirement of hand labor. Further, more consistency is possible in thetwist angle from the hub to the tip of the blade due to the eliminationof material warpage during fabrication of the propeller.

Also, the injection molded propeller presents a typical weight savingsof 25 percent when plastic is substituted for propellers made usingconventional methods and materials.

Although an injection molded propeller is discussed herein it should beappreciated that the propeller and the method of making the propellercan be used equally well in manufacturing different types of pulleys,flywheels and the like for securing a shaft to a hub with a positivelocking system as discussed herein.

The injection molded propeller and method of making the propeller isadapted for mounting on a drive shaft with the shaft having a pluralityof ribs axially disposed around the circumference of the shaft. Thepropeller includes a hub having a bore therethrough. A plurality ofpropeller blades are formed with the base of the blades integrallyformed and attached to the outer circumference of the hub. Grooves areaxially spaced around the inner circumference of the hub. The bore ofthe hub is adapted for receiving the shaft therein with the ribs of theshaft indexed and received in the grooves of the hub.

The advantages and objects of the invention will become evident from thefollowing detailed description of the drawings when read in connectionwith the accompanying drawings which illustrate preferred embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a rear view of a four-bladed injection molded propellermounted on a drive shaft.

FIG. 2 is a side view of the propeller.

FIG. 3 is an enlarged rear view of the hub of the propeller.

FIG. 4 is a rear view of the drive shaft.

FIG. 5 is a sectional view of a portion of the hub disposed around aportion of the drive shaft.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 the injection molded propeller is shown and designated bygeneral reference numeral 10. The propeller 10 as shown in the drawingsincludes four blades 12 disposed at 90 degrees from each other with thebase 14 of the blades 12 integrally formed and attached to the outercircumference 16 of a hub 18. It should be noted while four blades 12are shown it can be appreciated that various numbers of blades can beintegrally formed in the hub 18.

The hub 18 includes a bore 20 for receiving a shaft 22 therethrough. Thedetailed structure of the shaft 22 can be seen more clearly in FIG. 4.

In FIG. 2 a side view of the propeller 10 can be seen mounted on theshaft 22 with one end of the shaft 22 received inside the bore 20 of thehub 18.

In FIG. 3 an enlarged rear view of the hub 18 of the propeller 10 can beseen. In this view an inner circumference 24 of the hub 18 can be seenwith four grooves 26 axially spaced therearound and disposed at 90degrees to each other. Also the grooves 26 are disposed at a 45 degreeangle from a center line 13 of the blades 12 as shown in FIG. 1. Theconcept allows for equally spaced grooves at equal distance between thebladed or distributed mass, that is for a three bladed propeller thecorresponding angles are 120 degrees and 60 degrees. The grooves 26 areadapted for receiving axially extending ribs 28 formed around an outercircumference 30 of the shaft 22 as shown in FIG. 4. When the shaft 22is inserted inside the bore 20 of the hub 18 the ribs 28 are indexed forreceipt inside the grooves 26.

It should be noted that the injection molded propeller 10 is formedusing standard injection molding technique wherein a negative propellerdesign is used to produce a mold die for receiving standardthermoplastic molding materials such as DuPonts Rynite 545 and marketedunder this trademark. Also other types of thermoplastic material such aspolyester, polyethylenes and polypropylenes or the like may be used.This type of material is loaded with chopped fiberglass. The mixture isthen received in a hopper and fed into a heated screw and barrel of astandard injection molding machine.

The molding material is injected into the molded die using the heatedscrew and barrel. The molding pressures are between 500 psi and 2,000psi. After molding the hot propeller 10 it is removed from the mold die.As the thermoplastic material continues to shrink as it cools the driveshaft 22 is fitted into the hot propeller hub 18. As the propeller 10continues to cool it shrinks around the ribs 28 of the shaft 22 forminga high pressure bond of approximately 3,000 psi. This method ofattaching the drive shaft 22 to the hub 18 forms a positive locking hub18. Also by offsetting the grooves 26 at a 45 degree angle from thecenter line 13 of the propeller blades 12 the elastic deformation shapeof the hub 18 shown in dotted line 32 is at its greatest along thecenter line 13 and at its least in the mass of the hub 18 where thegrooves 26 are formed therein. Because the deformation due to therotational forces of the blades 12 is at its greatest along the centerline 13 of the blades 12, this deformation causes compression of the hub18 in the area of the grooves 26 and against the ribs 28 to complimentthe shrink fit provided by the chemical and thermal shrinkage.

In FIG. 5 an enlarged view of a portion of the hub 18 and the shaft 22is shown wherein stress relief voids 34 are formed during the molding ofthe propeller 10 to prevent forces due to thermoplastic shrinking androtational stress which causes cracking or structural weaknesses in thegroove area 26 in the hub 18 during the inserting of the shaft 22 in thehub 18 or during the operation of the propeller 10.

Changes may be made in the construction and arrangement of the parts orelements of the embodiments as described herein without departing fromthe spirit or scope of the invention defined in the following claims.

What is claimed is:
 1. A method of attaching a propeller having a hubwith a bore therethrough, the hub having a plurality of grooves spacedaround the inner circumference of the hub and propeller blades extendingoutwardly from the outer circumference of the hub, the stepscomprising:injection molding the propeller using a thermoplasticmaterial in a mold die; forming grooves spaced around the circumferenceof the hub offset from the center line of the blade in the mold die;removing the propeller from the mold die; fitting a drive shaft into thepropeller hub, the drive shaft having ribs disposed around thecircumference of the shaft, the ribs indexed with the grooves in the hubwhen the shaft is inserted therein; and cooling the propeller hub on thedrive shaft thereby providing a shrink fit and forming a high pressurebond therebetween, the grooves having a depth greater than the height ofthe ribs so that upon shrinking a space is left between the end of theribs and the grooves.
 2. The method as described in claim 1 whereinduring the step of injection molding the propeller further includingforming stress relief voids by enlarging the crown of the grooves spacedaround the inner circumference of the hub so when the hub is shrinkfitted on the shaft, voids are formed at the crown of the ribs of theshaft.
 3. A method of attaching a propeller having a hub with a boretherethrough, the hub having a plurality of grooves spaced around theinner circumference of the hub and propeller blades integrally formedand extending outwardly from the outer circumference of the hub, thesteps comprising:injection molding the propeller using a thermoplasticmaterial in a mold die and forming stress relief at the crown of thegrooves formed in the hub and forming the grooves offset in the hub fromthe center line of the integrally formed propeller blades; removing thepropeller from the mold die; fitting a drive shaft having a plurality ofribs disposed around the circumference of the shaft into the hub withthe ribs indexed with the grooves in the hub of the propeller; coolingthe propeller hub on the drive shaft with the hub shrinking as thethermoplastic cools and forming a high pressure bond therebetween,leaving a space between the ends of the ribs and the crown.
 4. Themethod as described in claim 3 wherein during the step of injectionmolding further including forming four propeller blades with the groovesin the hub formed at 45 degrees from the center line of the blades.